1. Field of the Invention
This invention relates to an apparatus for supplying steam to a vacuum chamber to perform an ashing process of a photoresist (exfoliation process of organic film) in a production process of semiconductor devices or the like and to a method for controlling the same.
In the production of semiconductor devices, liquid crystals, high polymer materials, ceramics or the like, gas phase plasma processes are sometimes used for treating the surfaces of work pieces in a vacuum or low pressure environment, because such processes are generally easy to control.
Often, however, the pressure of a gas suitable for use in such a process is not high enough. In particular the pressure of steam is low, i.e., about 20 Torr at room temperature and steam pressure very much depends on the temperature of the steam. Therefore, steam is a gas which is difficult to supply in large amounts and in a well controlled condition.
In the production of semiconductor devices, steam may be used as a reaction gas in a process for ashing resist or other resin films. Therefore, an apparatus for providing a stable supply of steam is needed so as to improve process accuracy.
The steam supplying apparatus is used to supply the steam from a closed water tank containing water via a mass-flow controller to a vacuum chamber.
In such steam supply systems it is desirable to supply the steam in a stable condition without raising the temperature of the portions of the pipe system not in the water tank, including the valves and the mass-flow controller, relative to the water temperature in the water tank, and without blockage of steam in the mass-flow controller.
The present invention is applicable to a steam supplying apparatus and to a control method therefor which satisfy the above-mentioned requirements.
2. The Prior Art Background
The steam generation chambers used in connection with the processes described above for generating steam to be supplied to a vacuum chamber are often made of quartz, which presents some drawbacks, such as uneven temperature and difficulty in precise control of evaporative flow, since quartz is easily damaged and does not have good thermal conductivity. Recently, metal chambers having their inside surfaces made of soda glass coated with enamel have become known. However, in such chambers, foreign substances (Na, Fe, Ca or the like) present in the glass may enter and contaminate the water.
In a steam supplying method, the steam is supplied as a result of the pressure difference between the saturated vapor pressure of steam (about 24 Torr at 25.degree. C.) and the pressure inside the chamber.
To keep the flow of steam supply constant, it is therefore necessary to keep the temperature of the steam generation chamber constant. However, in the prior art, temperature has been controlled using a mantle heater, which has its own limitations and has not been sufficient.
Also in the prior art, the steam sometimes deteriorates the pump oil of a rotary pump or the like and the deteriorated oil then remains in the vacuum chamber to reduce the vacuum. Thus, steam has become one of the most unfavorable gases for introduction into the vacuum chamber. As a result no method for positively introducing steam into a vacuum chamber has been developed.
FIG. 5 illustrates a system for positively introducing steam into a gas phase process. In this case a carrier gas is bubbled through water and the wet carrier gas is then introduced into the vacuum chamber.
FIG. 6 is a schematic diagram illustrating a prior art steam supply system.
The system of FIG. 6 includes a bubbler tank 41 filled with water, a vacuum chamber 42 where a gas process is performed, a mass-flow controller (MFC) 43, and a valve 44.
A carrier gas is introduced via the mass-flow controller 43 into the water in the bubbler tank 41 where water vapor for the gas process is picked up. The water vapor (or steam) is then introduced into the vacuum chamber 42 with the carrier gas.
In such a method, the amount of steam actually introduced into the process depends on the partial pressure of steam in the mixture of the carrier gas and the steam. In general, the partial pressure of the carrier gas is higher than the partial pressure of the steam, and the amount of steam is therefore not so much.
In order to introduce more steam, it is therefore necessary to use a large amount of carrier gas and a large air exhaust system is required to maintain the vacuum chamber in an evacuated condition.
In this method the variable range of the ratio between the carrier gas and the steam is limited.
Thus, in prior art apparatuses, it has been difficult to stably control the flow of steam because the steam always contains contamination. It also has been difficult to control the flow of steam to be introduced into the vacuum chamber.
A prior art steam temperature control system will now be described.
A conventional method for introducing steam to a vacuum system includes the use of a water tank, pipes and a mass-flow controller which are arranged so that a constant temperature is uniformly maintained and controlled throughout the system. However, this method has problems, such as, the constant temperature system itself is relatively large, the cost of the system is high, and it is difficult to control the temperature in the system.
A conventional method for solving such problems comprises controlling the amount of steam introduced into the system by controlling the water temperature in the water tank and elevating the temperature in other portions of the system and in the mass-flow controller so as to schematically prevent blockage.
A prior art system is illustrated in FIG. 7.
As shown in FIG. 7, the prior art system for supplying steam includes a steam supplying apparatus having a mass-flow controller. The illustrated steam supplying apparatus can be used in connection with a reduced pressure CVD (Chemical vapor deposition) process of the like. The system illustrated in FIG. 7 includes a water tank 31, temperature controllers 32a and 32b, valves 33a and 33b, a mass-flow controller 34, a heater 36 and a vacuum chamber 37.
As shown in FIG. 7, the steam flow from the water tank 31 is determined by the rate of evaporation of water and temperature of the water in water tank 31 is controlled with a double bath system using a temperature controller 32a. The pressure curve of water evaporation is relatively steep as compared with TEOS (Tetra ethyl orthosilicate; Si(OC.sub.2 H.sub.5).sub.4), and it is preferred to precisely control the temperature in the water tank 31 using the temperature controller 32a.
The temperature in the portions of the system outside the water tank 31, including the pipes 35, the valves 33a and 33b and the mass-flow controller 34, is raised by a tube heater 36 and the temperature controller 32b so that the steam is not blocked. The heater 36 is provided so as to entirely enclose the pipes 35, the valves 33a and 33a and the mass-flow controller 34, and a temperature controller 32b is provided, such as at the mass-flow controller 34, so as to control the temperature of the heater 36.
In the prior art steam supplying apparatus as shown in FIG. 7, the portions of the system not in the water tank 31, including the pipes 35, the valves 33a and 33a and the mass-flow controller 34, are entirely covered by the heater 36 and the temperature thereof is controlled by a single temperature control means 32b. Also, in this steam supplying apparatus, for example, a water temperature of 50.degree. C. in the water tank 31 is necessary to supply the steam at a flow rate of 600 cc/min. The pipe system including the pipes 35, the valves 33a and 33b and the mass-flow controller 34 must be kept at a temperature over 60.degree. C. by the heater 36 and the temperature control means 32b in order to prevent the steam from being blocked.
Also, a water temperature of 55.degree. C. in the water tank 31 is necessary to supply the steam at a flow rate of 900 cc/min., and in this case, the temperature of the portions of the system outside the water tank 31 must be over 80.degree. C.
If it is necessary to supply the steam at a flow rate of 1100 cc/min., the water temperature in the water tank 31 must be 60.degree. C. However, in this case, even if the temperature of the portions of the system outside the water tank 31 is raised to 90.degree. C. the steam is easily blocked at the mass-flow controller 34. Also, in this conventional steam supplying apparatus, depending on the structure and thermal capacity of the mass-flow controller 34, the temperature of the inner walls of the mass-flow controller 34, which are contacted by the passing steam, is a little lower than that of the front and rear parts thereof. As a result, there is a great possibility that condensation will occur.
Thus, if a large amount of steam is to be supplied, such as a flow of 1100 cc/min., the amount of evaporation from the water tank 31 increases as an exponential function, and therefore, the amount of steam delivered from the water tank 31 can be increased by increasing the temperature or the water surface area. However, to prevent the portions of the system outside the water tank 31 from being blocked with steam, there is the problem that the system pipes must be kept at a relatively high temperature. In particular, to heat the system pipes to a temperature over 100.degree. C. it would be difficult to use pipes formed from resin in view of its lack of resistance to heat. If metal is used in place of resin as the pipe material, the pipes will generally be subject to erosion, and when used in manufacturing semiconductor devices, metal pipes may cause a metal contamination. Accordingly the use of metal pipes is not appropriate.